Flicker Perimetry

Chris A. Johnson, Ph.D.

INTRODUCTION

The ability to detect
intermittent light and dark alternations of a visual stimulus (flicker or
temporal visual processing) is an important component of visual function
throughout the field of view. Rapid
changes in the luminance or contrast of a stimulus can be important for
detecting environmental changes, motion, and awareness of objects in peripheral
vision. A thorough description of the
variables influencing and mechanisms underlying flicker sensitivity is beyond
the scope of this presentation, but there are several references that can
provide a comprehensive review.1-5Flicker sensitivity has been a topic of interest to many investigators
for nearly 200 years.

For many years, psychophysical
flicker sensitivity has been reported to be diminished in glaucoma and ocular
hypertension.6 Tyler reported
that glaucoma patients demonstrated a
high temporal frequency flicker sensitivity loss, and up to 90% of ocular
hypertensives also exhibited a high temporal frequency deficit.6However, the ocular hypertensive cases were
actually the fellow eyes of patients who had glaucoma in the other eye, which
draws into question the likelihood of disease in the fellow eyes.Additionally, subsequent investigations have
reported that reductions in sensitivity with age are more prominent for high
temporal frequencies than for low and intermediate temporal frequencies.5
Thus, the high temporal frequency deficit may be related to the normal aging
process rather than a selective reduction in high frequency flicker sensitivity
that is related to glaucoma, and this has been confirmed in subsequent studies.7,8Nonetheless, it is important to recognize
that the ability to detect flicker is a sensitive and early indicator of
functional loss in glaucoma, and subsequent studies have confirmed this result.

Flicker perimetry is a visual
field test procedure that evaluates an observer’s ability to detect light/dark
stimulus alternations (flicker) at various locations in the field of view. In general, there are three types of flicker
perimetry test procedures that have been utilized: (1) contrast modulation
flicker, (2) critical flicker fusion (CFF), and (3) luminance pedestal
flicker. Contrast modulation flicker
uses a stimulus that is matched in luminance to the background.The contrast of the stimulus is then
modulated temporally according to a fixed frequency, and the amplitude of
flicker modulation needed for detection of the stimulus is determined (Figure
1a) for different rates of flicker.Critical flicker fusion (CFF) uses a sinusoidal grating with 100% (or
close to 100%) contrast, and determines the maximum frequency or rate of
flicker that can be distinguished from a steady, uniform field (Fig 1b).Luminance pedestal flicker presents a
flickering stimulus superimposed on a pedestal of steady light and determines
the amount of flicker that is needed to distinguish the flicker from a steady
uniform stimulus (Figure 1c). There are advantages
and disadvantages of each procedure, and a brief description of findings for
each procedure is presented below. Only
in a few instances have different methods of measuring flicker sensitivity
thresholds been compared.9 Methods other than the ones described above
have also been used in characterizing flicker perimetry.10-12

Contrast Modulation Flicker

As
described earlier, contrast modulation flicker perimetry is performed by using
a stimulus that is matched in luminance and color to the uniform background
which then undergoes a light and dark alternation (flicker) at a predetermined
frequency. The amplitude or contrast (modulation)
of flicker needed to detect the stimulus is then determined at key locations in
the visual field to yield a perimetric map of flicker sensitivity.When presenting this flickering stimulus, it
is important to avoid abrupt stimulus onsets and offsets (high temporal
frequency transients) that might affect the ability to detect the
stimulus. For this reason, many flicker
modulation stimuli are presented within a temporal cosine envelope or Hanning
window to minimize this possibility.Additionally, it is also important to use larger stimuli in order to
enhance reproducibility of the measurements and maximize the dynamic response
range. Although there has been debate
about which temporal frequencies provide the best information related to
detection of flicker,5,6 it appears that all temporal frequencies
are able to detect glaucomatous damage in a reasonable equivalent fashion as
shown in the figure on the previous page for stable and progressed early glaucomas
(EG) and ocular hypertensives (OH) that were tested with temporal frequencies
of 2, 8 and 16 Hz.7-9 A major
factor underlying the previous debate is related to the fact that individuals
with normal visual function demonstrate a greater reduction in sensitivity for
detecting high temporal frequencies in comparison to low and medium temporal
frequencies.5

Critical Flicker Fusion

Critical flicker fusion
perimetry (CFF) determines the highest flicker frequency that can be
distinguished from a uniform steady stimulus.Typically a fixed high contrast modulation is employed (near 100%
contrast). CFF perimetry is best
performed if the flicker modulation is about the average luminance of the
background adaptation level. Several
investigators have reported that this form of perimetry is superior to standard
automated perimetry in its ability to detect glaucomatous visual field loss and
evaluate the extent of glaucomatous visual field damage.13-19Additionally, it has been reported that this
form of flicker testing demonstrates minimal aging effects and is robust to a
variety of factors (e.g., blur) that have traditionally been difficult for many
other forms of perimetry. 13-15

Luminance
Pedestal Flicker

Many of the automated
perimeters in use today have a white hemispherical bowl that serves as the uniform
adaptation background and/or use light emitting diodes (LEDs) as the stimulus
light source. In these instances, it is
difficult to have a flicker stimulus that is identical in luminance and
chromaticity to the background. One
solution to this problem has been the development of a procedure in which a
flickering stimulus is superimposed on a luminance increment, and it is known
as luminance pedestal flicker.20-23 The observer’s task is to determine
whether the luminance increment is
flickering or is steady by pressing a response button when flicker is detected.
To date, only preliminary investigations of
this procedure in a perimetric context has been accomplished,22 and
evaluations of patients with ocular and neurologic disorders has not been
attempted with this technique. Future
assessments of the clinical utility of this procedure will be of great interest
to many practitioners and may yield additional information that will be of
assistance for diagnostic purposes.

One of the difficulties
associated with the use of different methods of performing flicker perimetry
concerns the selection of the most appropriate procedure for clinical
diagnostic purposes. Although a full
comparison of all procedures has not been performed to date, a direct
investigation of contrast modulation flicker and critical flicker fusion
perimetry has been performed in a group of participants with normal visual
function and a group of patients with glaucomatous visual field loss.9
A Receiver Operating Characteristic (ROC)
analysis was performed for both procedures to determine their sensitivity
(ability to detect damage due to glaucoma) and specificity (ability to
correctly classify individuals with normal visual function and no evidence of
glaucoma) for a large variety of different decision rules. It was also possible to perform both test
procedures using the same prototype automated flicker perimeter. In this manner, it was possible to evaluate
the clinical efficacy of each procedure in a manner that would permit them to
be directly compared. The figures to
the right indicate the ROC
curves for flicker perimetry conducted using flicker modulation (temporal
modulation perimetry or TMP) and critical flicker fusion (CFF) as the response
measure. For the average performance of
the full field (top graph), the curve is higher for TMP than for CFF, but this
difference is even greater when the average performance of visual field
quadrants is examined (lower graph).This reflects the importance of localized visual field loss for glaucoma,
and indicates that the TMP procedure is better able to characterize the visual
field damage produced by glaucoma.

As with most
clinical diagnostic test procedures, each of the methods of performing flicker
perimetry has advantages and disadvantages that must be considered.
TMP is more sensitive for detecting early
glaucomatous visual field loss, but the selection of appropriate target size,
background luminance, stimulus onset-offset conditions, test strategy, and
other properties of the test procedure are highly important. For example, by using an intermediate
temporal frequency that is near the peak of the temporal contrast sensitivity
function, test retest variability can be minimized, and the dynamic range of
the procedure can be maximized. CFF
perimetry can provide very useful information about the upper temporal
frequency limits of processing flicker information, although the variability
may be higher and the dynamic range may be limited. Additionally, CFF perimetry can be an easier
procedure to implement on existing instrumentation, although its use on video
monitor display systems would limit the possible temporal frequencies that
could be displayed. LPF perimetry is a
procedure that can be the most easily implemented on existing commercial
perimetric instruments. This makes its
availability a desirable characteristic.
However, the stimulus presentation consists of both a luminance onset
(pedestal) and the initiation of a flickering stimulus superimposed on the
luminance pedestal. For some subjects,
there may be confusion and errors produced when subjects are asked to respond
only to the flicker and not the stimulus onset.

Evaluation of
the influence of basic stimulus and test parameters on flicker sensitivity has
been examined by many investigators, along with procedures to optimize the
methodology for clinical testing of patients.1-23 These
investigations are important because small variations in pupil size, adaptation
level and may other features can dramatically alter flicker sensitivity.It is critical to apply test procedures that
are robust to non-pathologic influences on flicker sensitivity, and to
implement test procedures that are best designed to provide stable,
reproducible test results. In view of
the many stimulus parameters that can influence the sensitivity to flicker,
this represents a challenging and formidable task. However, many investigators have been able to
accomplish this goal in recent years.

From a
clinical perspective, flicker perimetry in its various forms has been reported
to be a sensitive indicator of early functional damage for a variety of
disorders, including age-related macular degeneration and retinal diseases,26-32
glaucoma, 6-10,13-19,24,25 and other ocular and neurologic
disorders. The figures below present examples
of visual field progression in the right eye of two patients with glaucomatous
visual field loss. Results are shown
schematically for nerve fiber bundle regions in the superior and inferior visual
field for normal (p> 0.05), and upper (high) and lower (low) 95 and 99%
confidence limits, designated as p<0.05 and p<0.01, respectively.
Five years of results are presented for
standard automated perimetry (W/W) and Short Wavelength Automated Perimetry
(B/Y) and three years of results are presented for TMP perimetry obtained for
flicker rates of 2, 8 and 16 Hertz. Note
that the deficits for TMP perimetry (especially at 8 Hz) are predictive of
future visual field loss for standard automated perimetry.

Additionally,
recent techniques have been developed that utilize flicker perimetry as a
method of testing the peripheral visual field of young infants.11,12
The use of flicker as a stimulus for evaluation of the peripheral visual field
of infants is particularly appropriate because it is one of a few stimuli that an
infant can attend to for prolonged periods of time.In this manner, important visual field
information can be obtained from this young population.

In summary,
flicker perimetry in all of its forms has made it possible to evaluate
peripheral visual function in an efficient manner, provides greater sensitivity
for detecting early pathologic changes, and provides the opportunity to
evaluate the visual field of individuals that otherwise may not be assessable.